The present invention relates to the production and processing of high T.sub.c superconducting bismuth-strontium-calcium-copper-oxide materials.
Since the discovery of the copper oxide ceramic superconductors, their physical and chemical properties have been widely studied and described in many publications, too numerous to be listed individually. These materials have superconducting transition temperatures (T.sub.c) greater than the boiling temperature (77 K) of liquid nitrogen. However, in order to be useful for the majority of applications at 77 K or higher, substantially single phase superconducting materials with high critical current densities (J.sub.c) are needed. In general, this requires that the grains of the superconductor be crystallographically aligned, or textured, and well sintered together. Several members of the bismuth-strontium-calcium-copper-oxide family (BSCCO), in particular, Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 (BSCCO 2212) and Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 (BSCCO 2223) have yielded promising results, particularly when the bismuth is partially substituted by dopants, such as lead ((Bi,Pb)SCCO).
Composites of superconducting materials and metals are often used to obtain better mechanical properties than superconducting materials alone provide. These composites may be prepared in elongated forms such as wires and tapes by a well-known process which includes the three stages of: forming a powder of superconductor precursor material (precursor powder formation stage); filling a noble metal container, such as a tube, billet or grooved sheet, with the precursor powder and deformation processing one or more filled containers to provide a composite of reduced cross-section including one or more cores of superconductor precursor material in a surrounding noble metal matrix (composite precursor fabrication stage); and subjecting the composite to successive physical deformation and annealing cycles and further thermally processing the composite to form and sinter a core material having the desired superconducting properties (thermomechanical processing). The alignment of precursor grains in the core ("textured" grains) caused by the deformation process facilitates the growth of well-aligned and sintered grains of the desired superconducting material during later thermal processing stages.
The general process, commonly known as "powder-in-tube" or "PIT", is practiced in several variants depending on the starting powders, which may be, for example, metal alloys having same metal content as the desired superconducting core material in the "metallic precursor" or "MPIT" process, or mixtures of powders of the oxide components of the desired superconducting oxide core material or of a powder having the nominal composition of the desired superconducting oxide core material in the "oxide powder" or "OPIT" process. General information about the PIT method described above and processing of the oxide superconductors is provided by Sandhage et al., in JOM, Vol. 43, No. 3 (1991) pages 21 through 25, and the references cited therein.
OPIT precursor powders are prepared by reacting raw powders such as the corresponding oxides, oxalates, carbonates, or nitrates of the metallic elements of the desired superconducting oxide. Because the OPIT precursor powder is formed by chemical reaction, its actual phase composition will depend on the quality and chemical composition of the starting materials and on the processing conditions, such as temperature, time, and atmosphere. Different processing conditions will give rise to different phases or different ratios of phases. If secondary phases, such as calcium plumbate (Ca.sub.2 PbO.sub.4), are formed in relatively large amounts, they can give rise to undesired effects. The presence of calcium plumbate, for example, disrupts the deformation induced texturing of the primary phase of the precursor powder, results in gas evolution during thermal processing, leads to growth of certain undesirable alkaline earth cuprate (AEC) phases which do not participate in the conversion of the precursor into the final oxide superconductor, and may induce undesired melting during heat treatments.
In order to avoid undesirable secondary phase formation, precursor powders sometimes are prepared by forming a BSCCO superconductor phase in a separate synthesis step and combining the BSCCO superconductor phase with a second metal oxide. The two powders may be readily reacted in a subsequent thermal processing step into the final oxide superconductor. By preparing the BSCCO superconductor in a separate reaction, it may be possible to avoid inclusion of undesirable secondary phases in the precursor powder.
A typical prior art preparation involves preparing essentially single phase (Bi,Pb)SCCO 2212 in a separate reaction and combining it with an alkaline earth cuprate. In subsequent thermal reactions, the two metal oxides react to form (Bi,Pb)SCCO 2223. The prior art reaction process is less than optimal because combining separate oxide powders necessarily reduces the intimate contact between the reactants (resulting in inhomogeneity), thereby requiring longer reaction times and/or harsher reaction conditions in order to obtain the final product. The slower reaction kinetics results in reduced control over the reaction process.
It is desirable, therefore, to have a method for preparing precursor powders having a controlled phase composition in a single step reaction process. It is further desirable to provide a method of controlling the phase composition of the precursor powder during its preparation and during subsequent thermomechanical processing.